This invention relates generally to injection molding and more particularly to a hot tip gated, sprue gated or edge gated injection molding system having two heated nozzles mounted in tandem, with a smaller one in front of a larger one.
The use of an elongated nozzle having an integral electrical heater for hot tip gating is well known in the art. Examples are shown in Gellert's U.S. Pat. No. 4,238,671 which issued Dec. 9, 1980 and U.S. Pat. No. 4,557,685 which issued Dec. 10, 1985. More recent examples are shown in Gellert's Canadian patent application Ser. Nos. 542,185 filed July 15, 1987 entitled "Coated Injection Molding Nozzle and Method" and the applicant's Canadian patent application Ser. No. 549,518 filed Oct. 16, 1987 entitled "Injection Molding Nozzle Having Grounded Heating Element Brazed into Pointed Tip." While these earlier systems are completely satisfactory for many applications, in some situations where it is desirable to have a relatively lengthy melt bore leading to the cavity misalignment at the gate due to thermal expansion and contraction has been found to be a problem. Furthermore, when molding thin walled parts with a relatively short cycle time, sensitive temperature control in the gate area is critical. Also, if a single nozzle is used with a long small diameter melt bore, melt pressure drops and shear stress due to rapid flow becomes unacceptably high.